The present invention generally pertains to an insulation sleeve for a galvanic primary element in the form of a round cell, and which is generally comprised of a coiled paper strip lined on one side with plastic, in addition to a process for producing it.
Primary cells are conventionally provided with an outer, sheet-metal casing which in addition to a protective function, also serves certain decorative purposes and provides required labelling for information such as battery type, voltage, manufacturer, etc. This metal casing must not have any electrical contact with other metallic parts of the cell which are in contact with the electrochemically active electrode substances, or which form a terminal of the primary cell. For this purpose, an insulation layer in the form of a cylindrical cardboard casing has long been used (e.g., in zinc-carbon primary cells) to protect the can-shaped negative soluble electrode from contact with the sheetmetal casing. To prevent leakage of the cell's electrolyte, the edges of the sheet-metal casing are sealed to a cover or base disk, generally by means of an asphalt compound or plastic.
However, such arrangements tend to exhibit certain disadvantages, resulting both from their production costs and their thickness. This latter disadvantage is particularly acute since the outer diameter of the primary cell is predefined, leaving less room for the active material. A further disadvantage of absorbent insulation materials such as cardboard or paper is that such materials are particularly capable of absorbing emerging electrolyte fluid, even directly from pores in the wall of a zinc can that has severely corroded.
A relatively expensive substitute for such cardboard casings is described in US-PS No. 2,802,042, which describes a three-layer plastic sleeve that is additionally laminated with Kraft paper or impregnated paper on both sides. Edges of this plastic sleeve which project above the electrode can are then folded together with the outer metal casing and with a base or cover plate. It has also been attempted to improve the shelf life and leak-proofing of round cells with insulated zinc anodes by shrinking a plastic tube (sleeve) onto the zinc can, and by providing ends of the can with insulation disks or rings.
In the interests of cost and efficiency, such cardboard and paper sleeves have been produced in a variety of different ways by coiling them, using helical winding machines, from endless webs of fabrics, cellulose derivatives and the like, which are then adhesively bonded to one another. An example of such processes may be had with reference to DE-PS No. 25 49 515. Processes for the production of such sleeves are also known from the packaging industry, where helical-winding and sleeve-sealing technology is widely applied.
DE-OS No. 1,956,779 describes an electrode sleeve for a round galvanic primary cell which is produced complete with a surrounding protective covering. This is accomplished by helically rolling a zinc strip onto a winding spindle (or mandrel), and then winding a double web of paper around the rolled zinc strip while simultaneously adhesively bonding the wound layers to one another.
However, experience has shown that even those helically wound insulation sleeves which have been manufactured according to the current state of the art are not capable of sufficiently retaining or immobilizing electrolyte which emerges from the soluble anode of the cell since they do not develop an insulation layer which is completely sealed. It has been found that even extensive adhesively bonded areas within a winding can neither prevent electrolyte from arriving, through leakage paths, at the outer sheet-metal casing of the cell, nor prevent corrosion currents from accelerating losses in capacity.